The present invention relates to a monoazo dye for thermal transfer printing, and especially to an imido-substituted monoazo dye for a sublimable magenta dye used in thermal transfer printing, which has good hue, gray and dye stability.
2. Description of the Related Art
In recent years, thermal transfer printing systems have been widely used in facsimile and copying machines, and have been further developed such that prints can be obtained from pictures generated electronically from a color video camera or computer. As one way of obtaining such prints, a thermal transfer printing system using a sublimable dye has been developed.
According to such a thermal transfer printing system, an electronically generated picture is first subjected to color separation by color filters. The respective color-separated images are then converted into electrical signals which are operated on to produce Y (yellow), M (magenta) and C (cyan) electrical signals. These signals are then transmitted to a thermal printer. Sublimable yellow, magenta and cyan dyes which are heat-transferable, are applied to a sheet-like substrate in the form of an ink, to form a transfer sheet. This is then placed in contact with the material to be printed, that is, a receiving sheet. The two sheets are then inserted between a thermal printing head and a platen roller. The thermal printing head hammer heating elements and is heated sequentially in response to the yellow, magenta and cyan signals. The transfer sheet is selectively heated in accordance with a pattern information signal corresponding to one color, so that dye from the selectively heated regions of the transfer sheet is sublimated and transferred to the receiving sheet and thereby forms a pattern thereon, the form and density of which is in accordance with the pattern and intensity of the heat applied to the transfer sheet. The process is then repeated for the other two colors, and by combining the three colors, a full-color hard copy is obtained which corresponds to the original picture viewed on a screen.
A sublimable dye for thermal transfer printing must satisfy several conditions. That is, such a dye should exhibit: (a) sufficient mobility so as to sublime while not thermally disintegrating during thermal printing head operation; (b) a high molar absorptivity coefficient; (c) stability with respect to light, humidity, heat and various chemicals; (d) good hue and gray characteristics; and (e) facility in manufacture.
As dyes for thermal transfer printing, U.S. Pat. No. 4,698,651 discloses a magenta dye-donor element comprising a substituted 5-arylazoisothiazole, U.S. Pat. No. 4,701,439 discloses a yellow dye-donor element having a cyanovinyltetrahydro-quinoline structure, and U.S. Pat. No. 4,695,287 discloses a cyan dye-donor element comprising a 2-carbamoyl-4- N-(p-substituted aminoaryl)imino!-1,4-naphthoquinone. Further, U.S. Pat. No. 4,764,178 discloses an azo dye having a diazotizable heteroaromatic amine and an aromatic coupling component. Also, Japanese Patent Laid-open Publication sho 59-78894 discloses a cyan dye having a naphthalene dione structure, and Japanese Patent Laid-open publication sho 59-227948 discloses a cyan dye having an anthraquinone structure. Many of these dyes, however, do not meet the requirements (a through e) for thermal transfer printing.
Furthermore, it would be desirable to improve stability with respect to light and heat as well as various color characteristics such as hue, and color development. To provide a magenta dye which has achieves such improvements, Japanese Patent Laid-open Publication sho 61-227092 discloses an azo dye having the following structure: ##STR2## wherein Y is hydrogen, alkoxy, methyl or halogen, and X is methyl, methoxy, formylamyl, alkylcarbonylamyl, alkylsulfonylamyl or alkoxycarbonylamyl. Similarly, Japanese Patent Laid-open Publication sho 62-99195 discloses an azo dye having the following structure: ##STR3## wherein Y is hydrogen, methyl or acylamyl, and X is cyano or halogen. However, neither of these azo compounds for magenta dye completely solves the above problems.